1. Field of the Invention
The present invention relates to a water current power generating device capable of generating electric power from water current power, and more particularly to the water current power generating device for generating electric power, by utilizing a vibration phenomenon in which a bowstring member or a bow-shaped member bends in a downstream direction, generates tension and sways in a bending and tensioned state, when put in a water current.
2. Description of the Related Art
As this kind of power generating device, a water current power generating device which converts ocean current energy into mechanical rotational energy is conventionally known, for example as described in patent reference 1 (JP Patent Application Publication No. 2007-009833).
The water current power generating device described in patent reference 1 is made of an axial flow water wheel (rotary wings) to be rotated by ocean current power, and a generating unit to generate electric power electro-magnetically by mechanical rotational energy of the axial flow water wheel.
However, with the water current power generating device (hereinafter, may be referred to as axial flow water wheel power generating device) described in patent reference 1, when a rotary sliding portion is put in sea water, electrolytic corrosion of a metal portion by sea water progresses, and also the sea water leaks from the worn rotary sliding portion, as the rotary sliding portion becomes worn, and as a result, there is a fear that a failure occurs in the axial flow water wheel power generating device. In addition, with such configurations of the axial flow waterwheel power generating device, there is a problem that a reduction of power generating efficiency occurs, since the rotary sliding motion becomes blocked, when marine species, suspended matter, and a like invade the rotary sliding portion.
Therefore, the axial flow water wheel power generating device is lacking in reliability and has a disadvantage that a device maintenance is required, since the axial flow water wheel cannot works continuously and smoothly over a long period of time in the sea and a like. Also, the axial flow water wheel power generating device has a weak point that the axial flow water wheel for an ocean current is expensive.
In addition, the axial flow water wheel power generating device is not suitable for uses in a tidal current or an ocean current shifting direction at frequent intervals, since it is difficult to locate the axial flow water wheel power generating device in the sea so that the axial flow water wheel is always directed against the tidal current or the ocean current.
As water current power generating devices for eliminating the above-mentioned disadvantages of the axial flow water wheel cower generating device, there are provided a water current power generating device utilizing a Karman vortex street, a water current power generating device utilizing a galloping vibration, a water current power generating device utilizing a fluttering vibration, and a like.
The water current power generating device utilizing the Kerman vortex street (hereinafter, may be referred to Karman vortex power generating device) is disclosed in patent reference 2 (JP Patent Application Publication No. 2001-157433) and patent reference 3 (JP Patent Application Publication No. 2010-136535), for example.
The Kalman vortex power generating device described in patent references 2 and 3, includes a column body (including a cylindrical body) and a pair of piezoelectric elements, and when a water current collides with the column body, is configured to convert electromagnetically or piezoelectrically a vibration caused by the Kalman vortex street into an electric output. The Karman vortex street is defined as two staggered rows of swirling vortices generated on both sides of the column body. With such configurations of the Karman vortex power generating device, an intricate mechanisms including an axial flow water wheel and a like becomes unnecessary.
Next, the water current power generating device utilizing the galloping vibration (hereinafter, may be referred to as galloping vibration power generating device) is disclosed in patent reference 4 (JP Patent Application Publication No. 2006-226221), for example.
The galloping vibration power generating device described in patent references 4 is made up of a galloping vibration body having a front plane which receives a fluid flow, and generating a galloping vibration perpendicular to a direction of fluid flow, a flexural vibrating body generating a flexural vibration based on the galloping vibration transmitted from the galloping vibration body, and a piezoelectric transducer converting flexural vibration energy into electrical energy.
Moreover, the water current power generating device utilizing the fluttering vibration (hereinafter, may be referred to as fluttering vibration power generating device) is disclosed, as a wind/water power generating device, in non-patent reference 1 (M. Yamagishi et al.; “Study on the fluttering characteristics of multi-articulated flat plate in the mean-flow”, Proceedings of the 43rd and 44th JAXA Workshops on “Investigation and Control of Boundary-Layer Transition”; Japan Aerospace Exploration Agency, pp. 41-44, February 2010), for example.
The fluttering vibration power generating device described in non-patent reference 1 includes a multi-articulated flat plate for vibrating in a fluttering mode in a mean wind/water flow, and a piezoelectric element for generating electric power from the fluttering vibration generated by the multi-articulated flat plate.
The multi-articulated flat plate is made up of a plurality of flat plates and a plurality of connecting shafts jointing the plurality of flat plates.
However, there are following problems with the Karman vortex power generating device disclosed in patent references 2 and 3. That is, for the Karman vortex power generating device to increase efficiency of power generation by achieving an efficient mechanical vibration, a resonant frequency of a column body must be adjust to match an occurrence frequency of the Karman vortices.
When both frequencies match, as shown in FIG. 4, a lateral vibration displacement “β” which occurs on the column body by the Kaman vortices reaches its maximum (“B” in FIG. 4). The lateral vibration displacement “β” is defined as a vibration displacement occurring on the column body in a direction orthogonal to a water current. When the lateral vibration displacement “β” reaches its maximum (“B” in FIG. 4), tension σ of the column body reaches its maximum, and an output voltage from the piezoelectric elements also reaches its maximum.
With respect to the Karman vortices, its occurrence frequency is uniquely determined by fluid properties, such as a density and viscosity of fluid, a current velocity, and a diameter of the column body.
On the other hand, With respect to the mechanical vibration of the column body, its resonance frequency is determined by material constants such as a weight of the column body, a diameter and length of the column body, the elastic modulus of a supporting body for supporting the column body, and a like. In addition, the column body is made of a metal and a synthetic resin with high rigidity and the resonance frequency of the column body is high and its sharpness of resonance Q becomes very high, since the column body is placed in a manner of both-ends supporting structure, or cantilever structure.
For the above reasons, it is not easy to design and manufacture the Karman vortex power generating device for uses in a sea area where current velocity varies with a lapse of time, so that the resonant frequency at the column body constantly matches the occurrence frequency of the Karman vortices varying in response to the current velocity.
Consequently, there is a problem that, for the Karman vortex power generating device to increase efficiency of power generation, a column body having a great diameter can be effectively used in fast-moving current area, but not in a slow-moving current area, whereas a column body having a small diameter can be effectively used in a slow-moving current area, but not in a fast-moving current area.
That is, with the Karman vortex power generating device, there is a disadvantage that an appropriate current velocity range is limited.
It is possible to extend the appropriate current velocity range sustaining resonance, utilizing a phenomenon called as a “lock-in”. However, an extendable current velocity range is at most two times.
The column body as mentioned above, if made up of a small the column member such as a bowstring, can be configured to achieve a desired resonance frequency by applying a desired tension. However, a tension applying mechanism is required to achieve the desired resonance frequency, and a complex tension controlling mechanism is also required to correct for tension variation, since the resonance frequency of the small diameter of the column body such as a bowstring varies with tension variation due to a fluid force as described later.
Moreover, in real operation, there is a fear that a surface smoothing of the column body is compromised for adhesion of marine life, floating matter and a like, and there is also a problem that a reduction of power generating efficiency occurs for the Kaman vortex power generating device.
Flow separation tram a smooth surface of the column body is an origin of Kaman vortices, and therefore fluid separation becomes responsible for originating a turbulent flow, instead of the Karman vortices, as loss of smoothing on the surface of the column body is advanced.
Furthermore, there is a problem that the Karman vortex power generating device also is not suitable for uses in a tidal current or an ocean current shifting direction at frequent intervals, since power generating efficiency varies and reduces depending upon the direction of water current, as in the case with the axial flow water wheel power generating device.
Because the Karman vortices are not generated, in a stable state, by tidal current flowing an upward or downward slanting direction particularly occurring in a shallow sea area.
Moreover, with the galloping vibration power generating device, there is a problem that a front plane of the galloping vibration body must be placed in a manner to be perpendicular to the direction of fluid flow, in order to generate galloping vibration.
In order to solve the above problem, the galloping vibration power generating device described in patent references 4 further includes a direction determination plate (weathercock plate) capable of moving around in response to variations in direction of fluid flow, so that the front plane of the galloping vibration body constantly faces directly the fluid flow, even if the direction of fluid flow varies. However nevertheless, the galloping vibration power generating device described in patent references 4 has disadvantages of increasing in structural complexity and manufacturing cost.
In addition, with the galloping vibration power generating device described in patent references 4, the flexural vibrating body must convert the galloping vibration generated by the galloping vibration body into the flexural vibration, and transmit the converted flexural vibration to the piezoelectric transducer. For this reason, the flexural vibrating body requires a costly material with great rigidity.
Moreover, with the fluttering vibration power generating device (wind/water power generating device) described in non-patent reference 1, there is a problem that the above-mentioned multi-articulated flat plate for generating fluttering vibration is complicated in mechanical structure, and it is difficult to keep performance withstanding over long periods of time against a variety of external variations, such as variations of current in direction and in flow rate.
Particularly important problem is vulnerability of particular components, such as a wearing down of the mechanical sliding portion of hinge mechanism for enabling the multi-articulated flat plate to flutter.
Moreover, with the fluttering vibration power generating device, there is also a problem that, when a plurality of the fluttering vibration power generating devices is located in close proximity, mutual influence between the fluttering vibration power generating devices cannot be ignored. Because there is a fear that irregular flow occurs in surroundings or lower reaches under the influence of fluttering vibration generated by a upstream multi-articulated flat plate.